Studies are in progress on the transport and metabolism of Vitamin A. These studies aim to define in chemical and physical terms the transport system responsible for the transport of vitamin A in plasma, and to explore its roles in health and disease. The complete amino acid sequence of human prealbumin has been determined. The prealbumin molecule is a stable tetramer, composed of four identical subunits, with overall molecular weight of approx. 54,980. The amino acid sequence of human retino-binding protein (RBP) is now under study. Some of the properties of the vitamin A transport system have been explored with physico-chemical techniques, particularly with polarization of fluorescence and velocity ultracentrifugation. These studies have been complemented by biological studies in the rat, and by clinical studies in humans. The studies in the rat aim to define the factors which regulate RBP production in the liver, and hence control vitamin A delivery to tissues. The tissue distribution of RBP in normal and vitamin A-deficient rats has been studied in detal. Clinical studies have continued to examine the effects of a variety of disease on the levels of plasma RBP and prealbumin in humans. Studies of patients with protein-calorie malnutrition have been conducted in collaboration with investigators in Cairo and Guatemala, and are continuing the collaboration with colleagues in Thailand.